The present invention relates generally to determining the state of charge of a battery, and more particularly to systems and methods determining the state of charge using confidence values.
Automotive technology is rapidly expanding in the area of finding alternatives to using gasoline as the primary source of energy in vehicle propulsion systems. Many of these advances utilize either a hybrid mechanical-electrical system that recaptures some of the mechanical energy from the combustion engine as stored electrical energy, or a fully-electric propulsion system, which eliminates the need for an internal combustion engine entirely. With these advancements, the storage and management of electrical energy in vehicles has become of particular importance.
State of charge (SOC) is a commonly-used measure of the amount of charge available in a battery relative to the battery's full capacity. In automotive applications that use fully electric or hybrid-electric propulsion systems, SOC measurements provide a useful indication of the amount of energy available to propel the vehicle. Similar to the information provided by a fuel gauge, a state of charge measurement can provide a driver of an electric vehicle with an indication of how long the vehicle may travel before running out of energy.
Traditional estimations of the SOC of a battery fall into two general categories: voltage-based approaches and current-based approaches. Voltage-based approaches typically make use of the mostly nonlinear relationship between the battery's voltage and state of charge. Measurement of a battery's voltage and knowledge of the battery's voltage-SOC profile can therefore be used to determine the present state of charge of the battery. Current-based approaches, in contrast, estimate the SOC of the battery by tracking the amount of current into and out of the battery. Integration of current measurements taken from the battery corresponds to the amount of charge that has either entered the battery or left the battery during a given span of time, leading to these techniques sometimes being referred to as “Coulomb-counting” techniques.
Voltage-based techniques suffer from a number of potential sources of error. The measured voltage of a battery is dependent on a number of factors including the temperature of the battery and the rest time of the battery relative to the battery's diffusion time constant. Another potential source of error exists for batteries that exhibit nearly flat voltage-SOC characteristics, such as with lithium-based batteries. For these types of batteries, the change in the battery's voltage with its state of charge may be very slight, making any uncertainty in the voltage measurement another potential source of error. Therefore, the tolerance of the voltage sensor itself may also be a significant source of error, for voltage-based SOC estimates.
Current-based techniques also suffer from a number of potential sources of error. A first potential problem with current-based techniques is that they rely on comparing the amount of charge into or out of the battery to an initial measurement. Therefore, inaccuracies in the initial measurement can present one potential source of error for the SOC estimate. A second potential source of error results from integrating the current readings: over time, any small amount of error present in the measurements becomes magnified by the integration process. For example, the tolerance of the current sensor may contribute to a difference between the measured and actual battery currents. This difference may continue to grow via the integration process, leading to an increasingly inaccurate estimation of the SOC over time. A third potential source of error is the battery's reference capacity itself, which depends on the temperature and lifetime of the battery. This value must be estimated, introducing additional sources of error. For example, U.S. patent application Ser. No. 13/107,171 filed May 13, 2011 entitled “SYSTEMS AND METHODS FOR DETERMINING CELL CAPACITY VALUES IN A MULTI-CELL BATTERY” discloses such an estimation technique and is assigned to the assignee of the present invention, the entirety of which is hereby incorporated by reference.
Recent efforts have been made to combine voltage-based and current-based techniques. For example, a voltage-based technique may be used when the SOC nears zero or one hundred percent, i.e., the battery is nearly empty or nearly full. When the SOC of the battery lies in the midrange, for example, between 20-90%, a current-based technique may be used to estimate the SOC. However, such a hybrid approach still suffers from the potential inaccuracies present when using either voltage-based or current-based techniques.